Development of multidrug resistance (MDR) to conventional and newer generations of anticancer therapeutics is a significant challenge in cancer therapy. MDR is presented clinically as a consequence of many factors that include poor systemic drug delivery efficiency, short residence time, poor permeability in tumor mass and intracellular availability, as well as microenvironmental selection pressures that give rise to resistant phenotype. As such, our innovative strategy to overcome MDR is based on development of combinatorial designed nano-formulation libraries with encapsulated small interference RNA (against mdr-1 and mrp-1 efflux transporter and Bcl-2 and survivin anti-apoptotic genes) and cytotoxic therapeutic agents (paclitaxel and doxorubicin). Our preliminary studies show that this multimodal therapeutic approach has significant potential in the treatment of refractory tumors. The specific aims of this project are: (1) to synthesize and characterize dextran-based macrostructures with fatty acids, thiol groups, poly(ethylene glycol) (PEG), and epidermal growth factor receptor (EGFR)- targeting peptide for combinatorial self-assembly in aqueous media into nanostructures that can encapsulate siRNA duplexes, paclitaxel, and doxorubicin; (2) high-throughput evaluation of cellular delivery and vesicular stability of siRNA duplexes and drugs, quantitative and qualitative gene silencing efficacy, cytotoxicity and apoptotic activity in wild-type (SK0V3) mdr-1 positive (SKOVSTR) human ovarian adenocarcinoma and wild- type (NIH-H69) and mrp-1 positive (NIH-H69AR) small cell lung adenocarcinoma cells; (3) selection of hits for in vivo evaluation of tumor targeting efficacy, residence, biodistribution profiles, and evaluation of non- compartmental pharmacokinetics in SK0V3TR and NIH-H69AR tumor xenograft models after systemic administration; (4) further refinement of hits and evaluation of in vivo gene silencing efficacy, tumor suppression, and inhibition of metastasis resistant tumor xenograft models after systemic administration of single and combination sIRNA/drug co-therapy; and (5) determination of acute safety profiles by measuring changes in body weight, blood cell counts, liver enzymes, and liver tissue histopathology with single and combination siRNA/drug co-therapy in resistant tumor models. The proposed clinically-translatable strategy holds tremendous promise in the treatment of refractory ovarian and small cell lung cancers, which continue to have very high mortality rates in the United States. RELEVANCE (See instructions): Tumor multidrug resistance (MDR) is a serious challenge in clinical cancer therapy. A multimodal approach that enhances drug delivery efficiency as well as overcomes cellular resistance is necessary to successfully treat MDR in cancer patients. In this study, we will develop a novel class of biocompatible dextran-based polymeric nano-assembled structures for encapsulation and delivery of small interfering RNA that can silence specific genes in resistant cells. and cytotoxic drugs for maximum cell-kill effect.